Is it posible to mix materials in fluid simulation
An alternative to the fluid simulation, could be using a particle system:


First add a colliding box:

Then add fluid sources (depends on the amount of balls of slime you want to use and what shape they are, in my example I use 2 cubes):

In the physics tab, enable Collision and set the Particle Damping Factor to 1 (highlighted with blue). Also make sure to set the end time (highlight green):

Then select a cube and add a particle system and tune the Emission settings highlighted in blue as you desire, but make sure to set Emit from: Volume:

Then tune Emitter geometry and Emitter object settings, but make sure to choose Physics: Fluid:

Next, select the other cube and add a new particle system by clicking the right most plus sign, then select the previous settings called ParticleSettings and click the plus sign next to it to make a copy:

Then tune the Emission, Emitter geometry and Emitter object settings, but make sure Emit from: Volume and choose Physics: Fluid  as in steps 4 and 5.
Then add two UV spheres and assign material to each of them:

Also enable Colission and make sure to set ParticleDamping Factor to 1:

Then select a cube, in the particlesystems settings: Render section, choose Object and select the name of the sphere and tune the size:

Then select the other cube and do the same:

After some modifications, for example choosing the Viscocity value to 8, you may obtain the following result: